The present invention relates to an electric melting furnace which can continuously melt the combustion residue such as ashes collected by dust collectors and the like and resulting from the burning of city refuse, sewage sludge and industrial wastes.
In general, most of the city refuse combustion residue, that is the ashes collected by the dust collectors and the like, are dumped. However, dumping places are getting scarce hence it is desirable that the volume of the city refuse combustion residue be decreased as much as possible.
Melting furnaces using oil burners have been developed to decrease the volume of the city refuse combustion residue but the furnaces now in use have various technical problems and therefore cannot be operated smoothly. For instance, in conventional furnaces, clinkers grow at the place where ashes are discharged, thereby making it difficult to continuously discharge the molten ashes. Further, it is not easy to continuously and completely melt the combustion residue, thus making the reduction in volume of the combustion residue difficult.
The present invention, in an attempt to solve the problems of the conventional melting furnaces which use oil burners or the like, uses an electric melting furnace for melting the combustion residue.
The electric melting furnace is widely used in the steel industry and other fields and is a technically stable apparatus, but operation of the conventional electric melting furnace requires experience and skill. When used in a plant in which the combustion residue is treated, many problems arise as follows.
Firstly, in order to discharge molten substances, the conventional electric melting furnace is either provided with a tapping opening on the furnace side wall which is intermittently unplugged to discharge the molten substances, or the declining method of discharge is used in which the whole structure of the electric melting furnace is tilted to discharge the molten substances from an exit positioned at the upper portion of the furnace.
The former method, i.e., the tap method, necessitates periodic unplugging of the tapping opening, and plugging again after discharging the molten substances. The operation of unplugging and plugging requires experience and skill and can be dangerous unless caution is exercised because the operation is usually conducted in a bad operational environment.
On the other hand, the latter method, i.e., the declining method has problems of poor thermal and melting efficiencies because the method necessitates interrupting the supply of electricity, lifting electrodes in the furnace and interrupting the feeding of the combustion residue while tilting the furnace.
Secondly, the conventional electric melting furnace has a problem in that molten metal sediments collect on the bottom floor of the furnace when the combustion residue, such as the refuse combustion ashes, contains plenty of metals. That is, the metals settle to the bottom floor since the specific gravity of the metals is greater than that of the molten slag (molten ashes). Since the depth of the total molten layer is generally fixed, the molten metal layer deepens (i.e. thickens) over a long period of operation while the molten slag layer shallows (i.e. thins). Since the lower end of the electrode is dipped and submerged in the molten slag layer, the lower end of the electrode is increasingly less submerged as the molten metal layer thickens, with the result that the load is lowered, thereby gradually reducing the melting capacity
In order to avoid the gradual deterioration in melting capacity, and thus to efficiently maintain the molten state, it is necessary to periodically discharge the molten metal in order to maintain the depth of the molten slag layer above a fixed value so that the depth of submersion of the electrodes is kept at a level providing the most desirable molten state. One of the methods for discharging the molten metals is to unplug a tap hole provided on the bottom of the furnace floor, but as stated above, this method requires experience and skill and can be dangerous if caution is not exercised.
The other method for discharging the molten metals is the declining method adopted with the conventional electric melting furnace, and, while the unplugging of the tapping hole is not necessary, the furnace body must be either tilted after lifting the electrodes out of the interior of the furnace body or tilted together with the electrodes placed in the furnace body. In the former case, electric loading is necessarily stopped while the electrodes are being lifted. Further, if the furnace has a furnace lid, the electrodes need be lifted well above the furnace lid with the result that the electrodes lifting distance increases, thereby making the electrode equipment large in size. On the other hand, in the latter case the whole furnace structure, including the electrode equipment, must be declined with the result that the declining apparatus must be large in size.
Thirdly, a problem relating to feeding of materials such as the combustion residue should be considered. The conventional electric melting furnaces adopt a batch method in which, after a fixed amount of materials is fed, the furnace discharges the whole molten substances, and then the furnace is refilled with a new supply of materials. But, the batch method has a limit to the melting efficiency and is not appropriate for the melting of combustion residue. That is, in order to achieve a stable and continuous melting and a continuous discharge of the molten substances for improving the melting efficiency, it is necessary to continuously and efficiently feed the furnace with the materials (combustion residue) and further to feed the materials evenly between the electrodes for obtaining the highest melting capacity at a speed which matches the melting speed.